A material including a nitride semiconductor such as GaN, AlN, InN or a mixed crystal of the nitride semiconductor has wide band gap and is used for high output electronic devices, short wavelength light emitting devices, or the like. Among the devices, technologies for a field effect transistor (FET), particularly, a high electron mobility transistor (HEMT) are being developed (see, for example, Patent Document 1). The HEMT using a nitride semiconductor is used for devices such as a high output/high efficiency amplifier and a high-power switching device.
One example of an FET using a nitride semiconductor is an HEMT that uses GaN as its channel layer and uses AlGaN as its barrier layer. In the HEMT, piezoelectric polarization occurs due to strain that occurs in the AlGaN of the barrier layer caused by the difference of lattice constant between AlGaN and GaN. The piezoelectric polarization causes high concentration 2 DEG (Two-Dimensional Electron Gas) to be generated in the channel layer. Another example of an FET using a nitride semiconductor is an HEMT that uses GaN in its channel layer and uses InAlN in its barrier layer for enabling high output. The use of InAlN in the barrier layer induces high concentration 2 DEG because InAlN has large spontaneous-polarization. Thereby, the HEMT using InAlN in its barrier layer allows a larger amount of drain current to flow compared to the HEMT using AlGaN in its barrier layer.